How to rotate (roll) the 3D view like I'm tilting my head?
It looks like you'd like to avoid viewport rotation lock around Z axis and to have possibility to rotate freely with RMB.

Option 1 - switch to Trackball rotating method

By default, Blender uses Turntable rotation method for manipulating view. It means the rotation in 3D Viewport will be locked to Z axis. See what the difference between Trackball and Turntable orbit styles.

To change that in File > User Preferences (or press Ctrl+Alt+U) choose Input page and find Orbit Style part.



After toggling it to Trackball the 3D Viewport will rotate freely.



Option 2 - use View Roll

Without changing anything in preferences it's possible to roll the view to change its angle. Hold Ctrl+Shift and rotate the mouse wheel(or by using the key shortcuts Shift+Pad4  / Shift+Pad6) to rotate left / right: